We investigate the electronic properties of the buckled honeycomb GaAs monolayer under a periodically circularly polarized laser (CPL) by first-principles calculations and Floquet theory. Under the irradiation of CPL with propagation direction in the $xy$ plane, a remarkable Rashba spin splitting (RSS) is induced in the valence and conduction bands at $\mathrm{\Gamma }$ point. The laser-driven RSS deviates from the classical linear Rashba model in a wide range of wave vectors, and can be interpreted by a nonlinear model involving the third-order terms. The phase diagram of Rashba effect with laser amplitude and propagation direction is worked out. The anisotropy of the nonlinear RSS is mainly determined by the laser propagation direction, providing the possibility of controlling the Rashba anisotropy characteristics. By carefully choosing the laser frequency and amplitude, giant anisotropic RSS can be obtained, which allows the generation of spin-polarized electrons via doping and facilitates the flexibility of spintronic devices.

Studying the strong correlation effects in interacting Dirac fermion systems is one of the most challenging problems in modern condensed matter physics. The long-range Coulomb interaction and the fermion-phonon interaction can lead to a variety of intriguing properties. In the strong-coupling regime, weak-coupling perturbation theory breaks down. The validity of $1/N$ expansion with $N$ being the fermion flavor is also in doubt since $N$ equals to 2 or 4 in realistic systems. Here, we investigate the interaction between $(1+2)$- and $(1+3)$-dimensional massless Dirac fermions and a generic scalar boson, and develop an efficient nonperturbative approach to access the strong-coupling regime. We first derive a number of self-consistently coupled Ward-Takahashi identities based on a careful symmetry analysis and then use these identities to show that the full fermion-boson vertex function is solely determined by the full fermion propagator. Making use of this result, we rigorously prove that the full fermion propagator satisfies an exact and self-closed Dyson-Schwinger integral equation, which can be solved by employing numerical methods. A major advantage of our nonperturbative approach is that there is no need to employ any small expansion parameter. Our approach provides a unified theoretical framework for studying strong Coulomb and fermion-phonon interactions. It may also be used to approximately handle the Yukawa coupling between fermions and order-parameter fluctuations around continuous quantum critical points. Our approach is applied to treat the Coulomb interaction in undoped graphene. We find that the renormalized fermion velocity exhibits a logarithmic momentum dependence but is nearly energy independent, and that no excitonic gap is generated by the Coulomb interaction. These theoretical results are consistent with experiments in graphene.

The Kitaev model realizes a quantum spin liquid where the spin excitations are fractionalized into itinerant Majorana fermions and localized ${\mathbb{Z}}_2$ vortices. Quantum entanglement between the fractional excitations can be utilized for decoherence-free topological quantum computation. Of particular interest is the anyonic statistics realized by braiding the vortex excitations under a magnetic field. Despite the promising potential, the practical methodology for creation and control of the vortex excitations remains elusive thus far. Here we theoretically propose how one can create and move the vortices in the Kitaev spin liquid. We find that the vortices are induced by a local modulation of the exchange interaction; especially, the local Dzyaloshinskii-Moriya (symmetric off-diagonal) interaction can create vortices most efficiently in the (anti)ferromagnetic Kitaev model, as it effectively flips the sign of the Kitaev interaction. We test this idea by performing the ab initio calculation for the candidate material $\alpha \text{−}\mathrm{Ru}{\mathrm{Cl}}_3$ through the manipulation of the ligand positions that breaks the inversion symmetry and induces the local Dzyaloshinskii-Moriya interaction. We also demonstrate a braiding of vortices by adiabatically and successively changing the local bond modulations.

${\mathrm{Ca}}_2\mathrm{Ru}{\mathrm{O}}_4$ is a compound with a Mott insulating ground state, which responds to external pressure in a variety of ways, some of which are expected (an insulator-metal transition) while others are more surprising (an expansion of the $c$ lattice constant). We provide here a comprehensive study of these pressure-induced structural and electronic changes using DFT$+U$ calculations, and demonstrate generally good agreement with experiment. The insulator-metal transition is reproduced and coincides with an isostructural transition, the $c$ lattice expansion, $\mathrm{Ru}{\mathrm{O}}_6$ octahedral distortions, and associated changes to the Ru $4d$ orbital order. The metallic part of the phase diagram features several competing phases. The high-pressure $Bbcm$ phase is found to be unstable in the ground state over a broad pressure range and suggested to be thermally stabilized instead.

We study dipole interseries transitions between the yellow and green exciton series in cuprous oxide including the complex valence-band structure. To this end, we extend previous studies of the spectrum of complex green exciton resonances [P. Rommel, P. Zielinski, and J. Main, Phys. Rev. B 101, 075208 (2020)] to optical transitions between different exciton states in addition to transitions from the crystal ground state. This allows us to augment the calculations on interseries transitions using a hydrogenlike model [S. O. Krüger and S. Scheel, Phys. Rev. B 100, 085201 (2019)] by a more comprehensive treatment of the valence-band structure.

Employing the ultrafast control of electronic states of a semiconductor quantum dot in a cavity, we introduce an approach to achieve on-demand emission of single photons with almost perfect indistinguishability and photon pairs with near ideal entanglement. Our scheme is based on optical excitation off resonant to a cavity mode followed by ultrafast control of the electronic states using the time-dependent quantum-confined Stark effect, which then allows for cavity-resonant emission. Our theoretical analysis considers cavity-loss mechanisms, the Stark effect, and phonon-induced dephasing, allowing realistic predictions for finite temperatures.

Weyl nodes and Fermi arcs in type-II Weyl semimetals (WSMs) have led to lots of exotic transport phenomena. Recently, ${\mathrm{Mo}}_{0.25}{\mathrm{W}}_{0.75}{\mathrm{Te}}_2$ has been established as a type-II WSM with Weyl points located near Fermi level, which offers an opportunity to study its intriguing band structure by electrical transport measurements. Here, by selecting a special sample with the thickness gradient across two- (2D) and three-dimensional (3D) regimes, we show strong evidence that ${\mathrm{Mo}}_{0.25}{\mathrm{W}}_{0.75}{\mathrm{Te}}_2$ is a type-II Weyl semimetal by observing the following two dimensionality-dependent transport features: (1) a chiral-anomaly-induced anisotropic magnetoconductivity enhancement, proportional to the square of in-plane magnetic field ($B_{\mathrm{in}}^2$); and (2) an additional quantum oscillation with thickness-dependent phase shift. Our theoretical calculations show that the observed quantum oscillation originates from a Weyl-orbit-like scenario due to the unique band structure of ${\mathrm{Mo}}_{0.25}{\mathrm{W}}_{0.75}{\mathrm{Te}}_2$. The in situ dimensionality-tuned transport experiment offers an alternative strategy to search for type-II WSMs.

In recent years twisted bilayers of two-dimensional (2D) materials have become very popular in the field due to the possibility to totally change their electronic properties by simple rotation. At the same time, in the wide field of photonic crystals, this idea still remains almost untouched, and only some particular problems have been considered. One of the reasons is the computational difficulty of accurate consideration of moiré superlattices that appear due to the superimposition of misaligned lattices. Indeed, the unit cell of the complex lattice is typically much larger than the original crystals and requires many more computational resources for the computations. Here, we propose a moiré-adapted Fourier modal method (MA-FMM) in the form of scattering matrices for the description of twisted one-dimensional (1D) gratings' stacks. We demonstrate that MA-FMM allows us to consider sublattices in close vicinity to each other and account for their interaction via the near field. In the developed numerical scheme, we utilize the fact that each sublattice is only 1D periodic and therefore simpler than the resulting 2D superlattice, as well as the fact that even a small gap between the lattices filters out high Fourier harmonics due to their evanescent origin. Such approach accelerates the computations from 1 up to 3 or more orders of magnitude for typical structures depending on the number of harmonics. In turn, the high computational speed paves the way for rigorous study of almost any photonic crystals of the proposed geometry and demonstration of specific moiré-associated effects.

We realize a magnetic skyrmion diode operated by a unidirectional skyrmion transport that flows in only one direction, which is highly significant for information processing in spintronic and nanoelectronic devices. We easily control the skyrmion transport by engineering asymmetric shapes of geometric structures. Here, we present a simple method to describe the underlying mechanism behind the unidirectional skyrmion transport by characterizing the topography of potential energy surfaces from a purely geometric perspective. Our approach enables a deeper physical insight into skyrmion transport manipulation and efficient design of skyrmion-based devices in geometric structures.

We show how a Dzyaloshinskii-Moriya interaction can be generated in an ultrathin metal film from a femtosecond pulse in electric field. This interaction does not require structural inversion-symmetry breaking, and its amplitude can be tuned depending on the amplitude of the field. We perform first-principles calculations to estimate the strength of the field-induced magnetoelectric coupling for ferromagnetic Fe, Co, and Ni, and antiferromagnetic Mn, as well as FePt and MnPt alloys. Last, using atomistic simulations, we demonstrate how an isolated antiferromagnetic skyrmion can be coherently nucleated from the collinear background by an ultrashort pulse in electric field on a hundred-femtosecond timescale.

The carrier-density distribution near a conducting interface and the related band structure are topics of great contemporary importance in low-dimensional quantum solids. We propose a scheme, innovatively combining the spectroscopy techniques of photoluminescence and time-correlated single-photon counting with transport measurements of resistivity to unravel the carrier distribution, the shape of quantum well, energy subbands, and Fermi surfaces of the conducting interface of ${\mathrm{LaVO}}_3$ and ${\mathrm{SrTiO}}_3$. Electronic parameters, such as the carrier density, the mobility, estimated from the electrical measurements, are in remarkably good agreement with those extracted from the spectroscopy with theoretical modeling providing a bridge between the two sets of data analysis.

We consider the ground state of an electron gas embedded in a quantum gyrotropic cavity. We show that the light-matter interaction leads to a nontrivial topology of the many-body electron-photon wave function characterized by a nonzero Berry curvature. Physically, this manifests as the anomalous Hall effect, appearance of equilibrium edge/surface currents, and orbital magnetization induced by vacuum fluctuations. Remarkably, closed analytical expressions for the anomalous Hall conductivity and macroscopic magnetization are obtained for the interacting many-body case.

A number of recent studies indicate that the charge conduction of the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface at low temperature is confined to filaments which are linked to structural domain walls in ${\mathrm{SrTiO}}_3$ with drastic consequences, for example, for the temperature dependence of local transport properties. We demonstrate that as a consequence of these current carrying filaments, on the nano-scale the magnetotransport properties of the interface are highly anisotropic. Our magnetoresistance measurements reveal that the magnetoresistance in different nanostructures ($<500$ nm) is random in magnitude and sign. Warming up nanostructures above the structural phase transition temperature (105 K) results in a significant change in magnetoresistance. Even a sign change of the magnetoresistance is possible. The results suggest that domain walls that are differently oriented with respect to the surface exhibit different respective magnetoresistances and the total magnetoresistance is a result of a random domain wall pattern formed during the structural phase transition in ${\mathrm{SrTiO}}_3$ at cooldown.

A discrete time crystal (DTC) is a quantum phase of matter related to the symmetries of a periodically driven system. It can appear when the initial state breaks these symmetries, and hence the observation of DTCs is intimately related to the symmetry of the initial state. In this paper, we investigate a nontrivial situation in which two interacting regions are simultaneously prepared in two initial states that preserve and break this symmetry. Specifically, we discuss the behavior of such a system under the influence of an environment by extending Floquet theory to the Liouvillian dynamics. Our analysis shows that the entire system evolves toward a DTC phase and is stabilized by the effect of dephasing. Our results provide a new understanding of quantum phases emerging from the competition between the coherent and incoherent dynamics in dissipative nonequilibrium quantum systems.

Honeycomb and kagome lattices exhibit extraordinary electronic properties. It is a natural consequence of additional discrete degree of freedom associated with a valley or the occurrence of electronic flat bands. The combination of both types of lattices, observed in CoSn-like compounds, leads not only to the topological electronic behavior, but also to the emergence of chiral phonon modes. We study CoSn-like compounds and show that chiral phonons are realized here. Previous theoretical studies demonstrated that the chiral phonons can be found in ideal two-dimensional honeycomb or kagome lattices. Recent experimental results support such a prediction as the chiral phonons were observed in the transition metal dichalcogenide ${\mathrm{WSe}}_2$. It turns out that in the case of CoSn-like systems with the $P6/mmm$ symmetry, the kagome lattice formed by $d$-block element is decorated by the additional $p$-block atom. As a result one finds a two-dimensional triangular lattice of atoms with nonequal masses and the absence of chiral phonons in the kagome plane. Contrary to this, the interlayer honeycomb lattice of $p$-block atoms is preserved and allows for the realization of chiral phonons. In this paper we discuss the properties of such chiral phonons in seven CoSn-like compounds and demonstrate that they do not depend on the atomic mass ratio or the presence of intrinsic magnetic order. The chiral phonons of $d$-block atoms can be restored by removing the inversion symmetry. The latter is possible in the crystal structure of CoGe and RhPb with the reduced symmetry ($P\overline{6}2m$) and in distorted-kagome-like lattice.

We study the nonlinear response of a ferrimagnetic sphere resonator (FSR) strongly coupled to a microwave loop-gap resonator (LGR). The measured response in the regime of weak nonlinearity allows the extraction of the FSR Kerr coefficient and its cubic damping rate. We find that there is a certain range of driving parameters in which the system exhibits instability. In that range, self-sustained modulation of the reflected power off the system is generated. The instability is attributed to absorption-induced heating of the FSR above its Curie temperature.

The origin of the pronounced and composition-dependent band-gap bowing in Sn/Pb mixed perovskite alloys has been under debate for a long time. Previous studies reported conflicting results on whether the chemical or structural effect is the dominant mechanism. In this paper, the band-gap bowing effect and its possible origins in recently synthesized two-dimensional (2D) ${\mathrm{Cs}}_2{\mathrm{Pb}}_x{\mathrm{Sn}}_{1\text{−}x}{\mathrm{I}}_2{\mathrm{Cl}}_2$ alloys are investigated from first-principles calculations. In agreement with experiments, a large and composition-dependent bowing coefficient is observed. By analyzing the contribution from volume deformation, charge exchange, structural relaxation, and short-range order, it is found that the dominant mechanism causing the anomalous gap bowing is the structural relaxation-induced wave-function localization, forming isovalent-defect-like states, despite the negligible octahedral distortion and small lattice mismatch between the two end compounds. This is understood by the s-p repulsion-induced strong antibonding character of the valence-band maximum which leads to a large deformation potential, thus even a small atomic displacement can result in a large shift of the energy level. These results thus highlight the critical role of strong deformation potential and structural relaxation effect in unusual band evolution of 2D Sn/Pb perovskite alloys, and can be helpful to the modulation of their band gap for optoelectronic applications.

Kitaev spin liquids (KSLs) contain both itinerant Majorana fermions and localized ${\mathbb{Z}}_2$ flux excitations. The mobile fermions transport energy, while the fluxes hinder heat flow. By controlling the flux configuration, one can separate a KSL into different domains via ${\mathbb{Z}}_2$ flux walls. We show that at low temperatures, the wall significantly suppresses the Majorana-mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and blocks the heat transfer between domains.